A polyurethane is generally prepared in bulk by reacting a polyether or polyester polyol of number average molecular weight 500 to 10,000, preferably 1000 to 4000 and typically 1,000 to 2,000, with an isocyanate and a low molecular weight "chain extender". If the chain extender is a diol, the polyurethane produced is thermoplastic and is typically referred to as a thermoplastic polyurethane (TPU), whereas if the chain extender has more than two hydroxyl groups per molecule or is an amine, then the polyurethane is a thermoset, typically referred to as a cast elastomer. The most common chain extender for TPUs is 1,4 butane diol. Butane diol is favored because it is an easily handled liquid, forms crystallizable hard segments, and is a commodity chemical. 4,4' methylene diisocyanate, MDI, is the most common isocyanate used in TPUs because it is linear which allows it to form high strength, crystalline hard blocks with butane diol. Toluene diisocyanate, TDI is typically used for foams and cast elastomers because it is a liquid, forms low viscosity prepolymers because of the difference in reactivity of the two isocyanate groups, and because it is less expensive than MDI.
Polyurethanes get their strength from the phase separation of the "soft phase", called the soft segment, from the "hard phase", referred to as the hard segment. Thus in standard urethane terminology the terms soft segment, which is the polyol, and hard segment, which is the sum of the isocyanate and chain extender, refer to the components used in making the polymer and not the physical composition of the phases in the final material. In reality the soft phase contains a substantial fraction of isocyanate and perhaps some chain extender in addition to the polyol.
Block lengths and hard segment contents are limited by the molecular weights of the starting materials. In the simplest example, a polyurethane which does not contain a chain extender, the hard segment content is determined by the molecular weight of the polyol. A TPU made from a 1,000 molecular weight (MW) polyol and MDI at a 1:1 NCO:OH ratio would have a hard segment content of 20%, whereas a 2,000 MW polyol would have a hard segment content of 11%. The addition of a chain extender then allows the adjustment of the hard segment without regard to the polyol molecular weight. However, since the hard blocks are normally very short, 1 to 4 "units" on average, phase separation is very sensitive to hard segment length which is coupled to polyol molecular weight. Therefore, for a given compatibility of the hard and soft phases, there is a hard segment content which provides optimal mechanical properties. Typically polyurethanes have hard segment contents between 30 and 50% because with the polyester and polyether polyols which are normally used, this level of hard segment content is necessary to achieve a composition with sufficient strength to be useful. This produces a range of materials that are typically harder than 70 Shore A. Soft, strong elastomers are difficult to design in these polyurethane systems because short hard segments are too compatible with the soft segments. It would be highly advantageous to be able to produce TPUs which are both soft and strong for use in applications which require this combination of attributes such as toys, sports grips, sealants, compounded rubbers, and adhesives. The present invention provides such soft, strong TPUs which have a hard segment content from 15 up to 30% by weight but still have a tensile strength of 1500 psi.
The molecular weight of the hard block distribution of the hard segment can also be strongly affected by the synthesis method. There are two standard methods of making a polyurethane: the one shot and prepolymer methods. In the one shot method the polyol, isocyanate, and chain extender are mixed together and cast into a mold, or reaction extruded to form pellets. Compatibility is extremely important in this method because phase separation and polymerization occur concurrently. If phase separation occurs too early, the polymerization will not be complete. An incomplete polymerization produces a low molecular weight polymer and leaves unreacted isocyanate groups which may lead to crosslinking either by thermally induced reactions or reaction with water. If phase separation occurs too late, the hard and soft phases will be poorly separated. This results in a high soft segment Tg, a low hard segment Tg (or Tm), high hysteresis, and poor tensile properties.
In the prepolymer method the polyol is reacted with part or all of the isocyanate. In the final step a chain extender, and more isocyanate if necessary, are mixed into the prepolymer and cured into the final TPU or cast elastomer. The prepolymer step may be the first step in an extrusion line, or the two steps may take place at two different locations. Here the compatibility requirements are somewhat reduced by the pre-reaction of the polyol with the isocyanate.
European Patent Application EP 0,732,349 describes thermoplastic polyurethane resins which are prepared by reacting an isocyanate, a polyol, a functionalized polyolefin, and a chain extender. The patent application describes both a one shot and a prepolymer process for preparing these compositions. In the one shot process, the polyol, terminally functional polyolefin, and chain extender were mixed together at elevated temperature for several hours and then the isocyanate was added. In the prepolymer method, the polyol and the terminally functionalized polyolefin were mixed together in a reactor and heated. Then the isocyanate was added and the reaction was allowed to proceed for about two hours. Finally, the chain extender was added to the mixture and allowed to react. We have found that the methods described in the European patent application produced TPUs with inferior tensile strength, elongation, and modulus. The present invention provides an improved process for producing mixed polyol TPUs which results in higher tensile strength and elongation and lower modulus.